1. Field
This invention relates to closed loop ion exchange processes. It is specifically directed to the prevention of compaction in the sorbent beds in such processes conducted in a simulated moving bed of sorbents.
2. State of the Art
Ion exchange systems which involve the passing of liquids containing at least two components through an ion exchanger are well known. Such processes are sometimes referred to as adsorber systems, and the process whereby one dissolved constituent is separated from another dissolved constituent by passing through an ion exchanger or adsorber bed is sometimes referred to as "adsorbtive separation". These processes generally involve passing a solution through a bed of resin, whereby one constituent is attracted to the resin bed, followed by an elution, or regeneration, step. The elution step removes the adsorbed constituent from the resin as "extract." The solution from which the adsorbed constituent has been removed is referred to as "raffinate," or sometimes "ashes." A common procedure is to contact the absorber bed (ion exchange resin) alternately with the feed stock (solution of constituents) and eluant, respectively, to achieve this separation. The feed stock and eluant may flow in either co-current or counter-current relationship through a stationary bed. Use of a stationary bed has limited the application of ion exchange separation techniques to batch operations.
Efforts have been made to conduct ion exchange processes in a fashion which simulates the characteristics of a continuous operation. One such approach has been physically to move the exchange resin, either continuously or by periodic pulsing, from one zone to another zone. Each zone is then operated continuously, either in an adsorber (or loading) cycle, or an elution (or de-sorbing) cycle. Mechanical wear, such as that caused by friction on the individual particles of resin, has been destructive. Accordingly, processes involving the physical movement of the ion exchange resin have not gained wide acceptance.
U.S. Pat. No. 2,985,589 (Broughton et al) discloses a continuous sorption process employing a stationary or fixed bed in a fashion which simulates a moving bed. Stationary beds operated by such procedures are commonly referred to as "pseudo-moving beds," or more often, as "simulated moving beds". More recent patents disclosing simulated moving bed processes include U.S. Pat. Nos. 4,182,633 (Ishikawa et al); 4,293,346 (Landis et al); 4,312,678 (Landis); and 4,319,929 (Fickel).
The simulated moving bed of the prior art represented by the aforementioned patents, generally is constructed as a single column partitioned into a plurality of individual compartments. The aforementioned Ser. No. 267,065 provides portions of the bed in discrete compartments connected for series flow in a closed loop. In either event, the individual compartments may be regarded as zones, connected in series with an inlet at the top of each zone and an outlet at its bottom. The process is regarded as continuous because a continuous circulation flow is maintained through the zones in series, the circulation liquid being collected at the bottom of the last zone in the series, after having percolated through each of the preceding zones in the loop. The collected liquid is re-introduced to the first zone of the series for recirculation. The inlets and outlets of each zone in the system are connected by means of an exterior manifold with appropriate valving to selectively introduce feed stock or eluant to the top of any zone, and to withdraw raffinate or extract from the bottom of any appropriate zone. Each zone may thus function in turn as the sorption zone, the displacement zone, the elution zone and the rinse (or regeneration) zone. The function of the zone is established by the nature of the medium which is either entering or leaving the zone at any particular moment. A complete cycle of the process is thus comprised of a series of modes of operation corresponding to the number of functions served in order by each zone.
It is an important objective in the operation of a simulated bed to maintain well defined interfaces between the various phases flowing through the system. Although the zones are ordinarily treated in batch fashion, the interface between adjacent phases progresses through the system continuously. An effort is made to correlate the opening of the manifold valve associated with an outlet (and the closing of the outlet valves of other zones) with the arrival of the front of the raffinate phase at that outlet. Opening of the appropriate outlet manifold valves is coordinated with the opening of inlet valves for the introduction of feed stock and eluant to the tops of the appropriate zones within the system. Introduction of these liquids is desirably done in a fashion which maintains an undisturbed interface between the liquid phases flowing through the column.
The simulated moving bed processes which have evolved suffer from a number of inherent shortcomings. For example, the use of a bed or ion exchange resin always requires periodic backwashing of the sorbent material, both for the removal of fines and to loosen the bed. After a period of operation, the inevitable compaction of the resin causes an intolerable pressure drop across the bed. A compacted bed impedes the percolation of liquid through the column. Moreover, it is important that the beds confined within each zone of a column be classified periodically into layers of equal particle size. Otherwise, it is impractical to maintain approximately equivalent conditions within each zone. It is also important periodically to remove entrained gas pockets from within the bed, because they tend to disturb the desired even cross-sectional fluid flow through the bed. According to conventional practice, when backwashing has become critically important, the entire bed has been removed from the column and replaced. Recently, several procedures have been suggested to avoid this requirement, but none of these procedures is entirely satisfactory.
U.S. Pat. No. 4,001,113 discloses an ion exchange treating system in which two or more exchangers or absorber vessels are connected in series, and each vessel is filled with ion exchange resin leaving sufficient free board to allow for expansion of the medium. The process disclosed does not involve a simulated moving bed, but the necessity for periodic backwashing is nevertheless present. Each vessel is provided with an expansion chamber positioned directly above it to accept resin during backwashing procedures. Each vessel is also provided with distribution systems at the top and bottom for the introduction or withdrawal of liquids. The aforementioned Ser. No. 267,065 discloses a simulated moving bed process in which each zone comprises one or more discrete vessels. Each of the vessels may be provided with a port at its top which communicates through valving to an expansion chamber. The expansion chamber functions as a receiver for backwashed compacted sorbent bed so that the bed of each vessel may be periodically fluidized and reclassified. This arrangement avoids the inconveniences attendant to maintaining the bed in a tall column. Several zone vessels may communicate with a single expansion chamber. The vessel being backwashed is non-operational from the standpoint of the process. That is, none of the specific functions of the process is performed in the vessel during backwashing. Accordngly, backwashing is done as infrequently as conditions will allow.
U.S. Pat. No. 4,293,346 discloses a backwashing procedure of sorts for simulated moving bed processes. The procedure suggested by that patent involves isolating a portion of the sorbent bed in a non-operational zone, and subjecting the isolated portion to continuous backwashing during a complete mode of operation. In effect, an additional zone is imposed on the process. The location of the backwashing zone is advanced along with the operational zones as the cycle is advanced through the several modes of the process. Although the process cycle can continue during backwashing, a portion of the sorbent bed is always in a non-operational condition.
An alternative procedure is suggested by U.S. Pat. No. 4,319,929, according to which circulation through a column is entirely stopped between the modes of a cycle while backflushing is conducted in the zone which has just served as the adsorption zone. This approach permits backwashing to occur over a relatively shorter period of time and utilizes the entire sorbent bed simultaneously in operational zones, but efficiency is negatively impacted by intermittently interrupting circulation.
Successful operation of a simulated moving bed process depends upon the maintenance of steady state equilibrium, as reflected by the absence of drift in the concentration gradient of the various components to be separated and in the fractions collected from the circulating loop. As previously indicated, it is important to maintain well defined fronts for the various phases flowing through the loop. It is also important reliably to predict the progress of these fronts through the loop. This prediction is correlated both to the establishment of a circulation flow rate within the loop and to the timing of shifting the opening and closing of the inlet and outlet ports connected to the manifold system. Traditionally, establishing both the circulation flow rate and the timing of the manifold flow control have been based upon either trial and error or complicated measurements of component concentrations. The aforementioned U.S. Pat. No. 4,182,633 discloses one approach to controlling a simulated moving bed process which involves measurements and rather complex computations. The aforementioned Ser. No. 267,065 discloses a reliable method for controlling a process without elaborate computations by fixing the circulating flow rate through a series of discrete vessels and then establishing cycling frequency in accordance with a known mathematical relationship based upon the flow rate of the non-sorbed component. In practice, the backwashing procedures suggested for the process, while helpful, are in need of further enhancement. Unfortunately, the other procedures heretofore known, including those of U.S. Pat. Nos. 4,293,346 and 4,319,929, are incompatible with the procedures of Ser. No. 267,065.
The disclosures of all of the aforementioned patents are incorporated by reference, and are considered to be generally instructive concerning the art of simulated moving bed processes.